Between the coming wave of new infrastructure spending, the growing concern about climate change, and, of course, the gnawing fear that gasoline prices will cruise back upward once the global economy climbs out of its sinkhole, we'll likely hear a lot more about public transit in the coming years. But it's rarely a straightforward subject—sure, at first, it sounds harmless enough to build a new light rail or subway system, but inevitably the objections start pouring in. Earlier this morning, I sat in on a heated discussion at the Brookings Institution on the proposed Purple Line in Maryland. The line itself is of mostly local interest, but many of the debates involved are relevant for any metro region grappling with new transit, so why not do a little recap? First, a map:

      

The logic undergirding the Purple Line is that D.C.'s Metro, like most old-school subways, is a hub-and-spoke model, built for an era when people lived in the suburbs and commuted downtown for work. Nowadays, though, most traffic flows from suburb to suburb—hence the need to interlink Montgomery County and Prince George's County. Most area residents favor some sort of connecting line; the bickering is over the details. Marc Elrich, a Montgomery County councilman, explained that he was agonizing about whether Maryland should spend $1.2 billion on a fixed light-rail system projected to transport 64,000 people per day, or spend just $600 million on a bus rapid transit (BRT) system with a dedicated lane, projected to transport 58,000, and use the savings for other worthwhile initiatives.

Chris Leinberger, a Brookings expert on development who comes at things from a real-estate perspective, countered that Elrich was approaching this too narrowly. Leinberger argued that transportation tends to drive development, and that transit projects should be viewed as a means of creating new value in a metro area. In that vein, he argued that middle-class people like trains well enough, but often refuse to ride buses, which carry the stigma of poverty; as a result, developers are much more likely to invest around rail stations than bus stops. (This may not be an ironclad law, but, alas, the United States has relatively few examples of successful BRT, a la the famous system in Curitiba, Brazil).

What's more, Leinberger assured the audience, developers will flutter to new light-rail stops in droves, because there's colossal pent-up demand in this country for transit-oriented development. By his count, some 30 to 50 percent of residents in U.S. metropolitan areas want to live in a walkable urban environment—a trend fueled by the growing number of single and childless couples, who will constitute 88 percent of household growth through 2040. Trouble is, he estimates there are currently only enough walkable neighborhoods to satisfy about 5 to 10 percent of metro residents, which is why rents in transit-accessible areas are so exorbitant. (Incidentally, the boom in childless households is one reason why development in D.C. could start to expand beyond Montgomery County and toward the northeastern suburbs, which have long been hampered by relatively inferior schools.)

Of course, to fix all this, new rail lines alone won't suffice. The towns around the proposed stops will have to revamp their zoning codes to allow high-density development near train stations—a suggestion that's typically greeted by angry, pitchfork-wielding mobs. (Ryan Avent recently dredged up a perfect example.) Now, since these changes in land use can both reduce greenhouse-gas emissions and bring down the cost of housing, Leinberger argued that environmentalists and social-justice activists should be at the forefront here. "Instead," he said, "you've just been leaving it up to developers—and no one seems to trust us!" Not that developers will ever be irrelevant: One interesting point Leinberger made was that if transit really does create the sort of value he expects, then real-estate developers should be more willing to pitch in and help finance these projects.

That still leaves the question of federal funding. Maryland has been working with the Federal Transit Administration to get these transit projects approved and shake loose matching funds, but John Pocari, the state's transportation secretary, observed that the process itself can be maddening. Notably, mass-transit projects must always prove that they're "cost-effective" over a fairly narrow timeframe. Yes, a transit project might be great for reducing carbon-dioxide emissions or essential for boosting the supply of affordable housing, but if it can't prove its own short-term cost-effectiveness, it'll never get off the ground. (Moreover, many cost analyses are often misguided, as one official in the audience point out: A bus system, for example, may appear to cost less upfront than light rail, even if the fact that it requires more drivers and frequent bus replacements means higher operating costs over the long haul.) Needless to say, highways rarely endure such scrutiny—indeed, for the full list of federal-funding biases that favor roads over transit, see this old post.

What's needed, Pocari concluded, is a more holistic approach to transportation that combines a broader vision of land-use planning with individual transit projects. The upside, he added, is that the transit funding process could be changed fairly easily by the Transportation Department itself—this doesn't need to wait for complex legislative fixes. That's why it's a genuinely encouraging sign that, for instance, veep-to-be Joe Biden recently came out and bragged about his "pro-rail bias" and talked up the need for more transit spending. An attitude like that can trickle down and have real consequences in short order.

--Bradford Plumer